This invention relates to a stationary electric device using a linear phenylmethylsilicone oil as an impregnating oil such as an insulating oil, a dielectric solution etc.
As a dielectric solution for capacitors or an insulating oil for transformers, there have long been used a PCB (polychlorinated biphenyl) series oil. This material, however, raised a serious environmental contamination problem and it has long been desired to develop a new insulating medium as a substitute for PCB. As one of such materials, a fire retardant silicone oil, particularly a dimethylsilicone oil, is considered. The dimethylsilicone oil is a linear polysiloxane synthesized, by a conventional method, using dimethyldichlorosilanes and trimethylmonochlorosilanes as raw material. The oil has a kinematic viscosity of 50 centistokes at 25.degree.C and a pour point of -55.degree.C. It is true that the dimethylsilicone oil has an excellent heat-resistance as well as a superb electrical property, but its side-chain methyl radicals are relatively easily decomposed under a large current arc discharge or a high electric field, producing the combustible gases such as a hydrogen gas, a methane gas etc. with the resultant disadvantage. The produced gas tends to be dissolved in the silicone oil. Some of the dissolved gas is separated from the silicone oil to create minute voids in the silicone oil and there is a danger that an electric device is dielectrically broken down due to a corona discharge under a high electric field. In a high-tension capacitor formed by combining dimethylsilicone oil used as an impregnating oil, with a capacitor element using as a dielectric, for example, a biaxially oriented polypropylene film or a combination of a polypropylene film with a capacitor paper, a corona starting voltage (CSV) and corona eliminating voltage (CEV) are low and a ratio of CEV to CSV is small. Consequently, a greater average voltage gradient can not be taken, even as compared with a capacitor using a mineral oil series, and the volume of the capacitor is unavoidably bulkier and no great merit is obtained from an economical viewpoint.
Furthermore, the corona level of a stationary electric device, such as a capacitor etc., using dimethylsilicone oil as an impregnating oil is relatively low. Therefore, when such a device is used for a long period of time, no high reliability is obtained.